During the production of semiconductor devices, leadframes are traditionally used as a cost-effective way to mount and process a plurality of semiconductor dice or chips concurrently. Each leadframe typically has a plurality of die pads for mounting the said chips. The leadframe also acts as a means to electrically connect the chip to external devices via leads of the leadframe. Bonding wires are connected to electrical contacts found on the chip and said leads of the leadframe in a process known as wire bonding. The wires usually comprise gold, aluminum or copper material.
After the chips are mounted onto the leadframe and the bonding wire connections are made between the chips and the leadframe, each chip has to be protected from the environment by encapsulating it with a plastic molding compound, such as epoxy molding compound (“EMC”). Each encapsulated chip constitutes a semiconductor package. The multiple semiconductor packages are then singulated or cut to form individual semiconductor devices.
It is important that adhesion between the leadframe material and the molding compound is strong. In the case of copper-based leadframes, adhesion may be reduced due to a layer of loose and uncontrolled native copper oxide forming on the surfaces of the leadframe. The problems associated with this occurrence include delamination of the encapsulant, liquid trapped beneath the encapsulant during packaging and/or liquids seeping or vapors condensing under the encapsulant, leading to mold failure. These problems are addressed in various ways in the prior art. For example, the problems are described in U.S. Pat. No. 4,946,518 entitled “Method for Improving the Adhesion of a Plastic Encapsulant to Copper Containing Leadframes”. This patent teaches improving the adhesion of plastic encapsulants to copper leadframes by exposing the copper leadframes to an active oxygen ambient at temperatures below the leadframe annealing temperature. This strengthens any native copper oxide present on the leadframes without increasing the thickness of the leadframes.
In another example, in U.S. Pat. No. 5,459,103 entitled “Method of Forming Lead Frame with Strengthened Encapsulation Adhesion”, the leadframe is plated with copper strike and the copper strike is exposed to an oxidizing agent to form a layer of cupric oxide (CuO) that promotes adhesive bonding between the plastic mold and the leadframe.
There are certain types of leadframe widely used in the industry that have multiple layers of material plated onto the leadframe, such as pre-plated frames (“PPF”). A three-layer leadframe plating scheme comprising nickel (Ni), palladium (Pd) and gold (Au) has been widely adopted in the market for over ten years as a green solution for semiconductor packaging. However, the uppermost layer comprising gold plating has lower adhesion strength to EMC than the underlying copper (Cu) material used as a typical leadframe base material. There is an increasing demand from the market for adhesion enhancement of EMC to such nickel-palladium-gold PPF leadframes so as to minimize package delamination.
One present method for adhesion enhancement of EMC to such PPF leadframes is to selectively electroplate nickel, palladium and gold onto certain areas of the leadframe surface by using a mechanical mask or a photo-resist mask, such that the leadframe base copper material can give better EMC adhesion than the uppermost gold layer. However, such an approach increases costs because there is a need to conduct repeated masking to selectively plate each layer of material only at certain portions of the leadframe, while leaving the rest of the base material unplated. Furthermore, by repeatedly applying and removing masks, there is a possibility of misalignment during each application, potentially causing quality issues or functional failure.
When using other masking methods such as using a photo-resist mask (negative dry film) instead of mechanical masking, the disadvantage is that masking is usually done prior to formation of the leadframe pattern by either stamping or etching. As a result, there will be exposed copper on the side walls of the external leads, which are more prone to solder non-wetting. On the other hand, when using a photo-resist mask (negative wet film), the disadvantage is that development of photo-resist on the side wall of the leads by UV exposure is very difficult, and thus the side wall of the lead may again be subject to solder non-wetting. It is therefore very hard to maintain a consistent quality.
In an alternative approach, U.S. Pat. No. 6,953,986 entitled “Leadframes for High Adhesion Semiconductor Devices and Method of Fabrication” discloses the selective plating of a noble metal in the form of thick palladium onto some areas of the leadframe while allowing the rest of the leadframe to have nickel diffusing up to the surface of the leadframe to form nickel oxide. Nickel oxide is believed to have better EMC adhesion than typical nickel or palladium. Nevertheless, the high cost of palladium and thus the expense associated with the need to plate palladium to about 70-80 nm thick on each leadframe is a deterrent.